The devastating Indian Ocean tsunami of 2004 and the Tōhoku earthquake in Japan in 2011 highlighted some of the worst-case scenarios for natural disasters. But humanity has not had to deal with a cataclysmic volcanic disaster since at least 1815, when the eruption of Tambora in Indonesia killed tens of thousands of people and led to a ‘year without a summer’ in Europe and North America. Such world-altering blasts rank at 7 or more on the Volcanic Explosivity Index (VEI) scale of eruptions, which goes to 8.

“The next VEI-7 eruption could occur within our lifetimes, or it could be hundreds of years down the road,” says Chris Newhall, a volcanologist with the Mirisbiris Garden and Nature Center in Santo Domingo, Philippines. But the time to have this discussion is now, he observes in Nature, so that researchers and government officials can plan and prepare before an emergency strikes.

Newhall is the lead author of a paper published last week in Geosphere1 that explores the potential consequences of the next VEI-7 eruption. His co-authors are volcanologist Stephen Self of the University of California, Berkeley — with whom Newhall devised the VEI scale2 in 1982 — and Alan Robock, a climate scientist at Rutgers University in New Brunswick, New Jersey. All three have researched the VEI-5 eruption of Mount St Helens in Washington state in 1980, and the VEI-6 eruption of Mount Pinatubo in the Philippines in 1991.

Those events killed dozens to hundreds of people and disrupted entire regions. Pinatubo even spewed enough sulfur dioxide into the stratosphere to cause global cooling. But a VEI-7 eruption would be of an entirely different scale. In 1257, a VEI-7 eruption in what is now Indonesia probably cooled the planet down enough to kick off a centuries-long cold snap called the Little Ice Age, Robock says. “These things are hugely important for the planet, but the next one will take place in quite a different environment,” he adds.

Newhall’s team says that researchers should start to prepare for a VEI-7 eruption by studying potential effects on crucial communications links — such as how atmospheric moisture and volcanic ash can interfere with global positioning system signals. Others could work to improve their understanding of how large amounts of magma accumulate and erupt, helping scientists to forecast where the next VEI-7 event might occur.